Migrate e2e tests to transaction-based isolation

Replace table truncation with transaction rollback for test isolation.
Each test now runs in a transaction that auto-rolls back, improving
test performance and isolation. Tests that call procedures with internal
transactions use getSharedDb() directly with appropriate comments.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api-server/src/__tests__/helpers/test-db.ts

@@ -238,3 +238,64 @@ export async function createTestUser(
 export async function destroyTestDb(db: Kysely<Database>): Promise<void> {
   await db.destroy();
 }
+
+// ============================================================================
+// Shared Database Singleton (for transaction-based test isolation)
+// ============================================================================
+
+let sharedDb: Kysely<Database> | null = null;
+
+/**
+ * Initialize the shared test database once.
+ * Runs migrations and truncates all tables to start with a clean slate.
+ * Subsequent calls return the existing connection.
+ *
+ * Use this with `withTestTransaction()` for fast test isolation.
+ *
+ * @example
+ * ```typescript
+ * beforeAll(async () => {
+ *   await initTestDb();
+ * });
+ *
+ * test("does something", async () => {
+ *   await withTestTransaction(getSharedDb(), async (db) => {
+ *     // test code using db
+ *   });
+ * });
+ * ```
+ */
+export async function initTestDb(): Promise<Kysely<Database>> {
+  if (!sharedDb) {
+    await runMigrations();
+    sharedDb = createTestDb();
+    await truncateAllTables(sharedDb); // Clean slate once at start
+  }
+  return sharedDb;
+}
+
+/**
+ * Get the shared test database connection.
+ * Must call `initTestDb()` first.
+ *
+ * @throws Error if database not initialized
+ */
+export function getSharedDb(): Kysely<Database> {
+  if (!sharedDb) {
+    throw new Error(
+      "Test DB not initialized. Call initTestDb() in beforeAll first.",
+    );
+  }
+  return sharedDb;
+}
+
+/**
+ * Destroy the shared test database connection.
+ * Call this in a global afterAll if needed.
+ */
+export async function destroySharedDb(): Promise<void> {
+  if (sharedDb) {
+    await sharedDb.destroy();
+    sharedDb = null;
+  }
+}

apps/api-server/src/__tests__/helpers/test-transaction.ts

@@ -0,0 +1,60 @@
+/**
+ * Transaction-based test isolation helper
+ *
+ * Wraps test code in a transaction that auto-rollbacks, providing
+ * fast test isolation without truncating tables between tests.
+ */
+
+import type { Database } from "@reviq/db-schema";
+import type { Kysely } from "kysely";
+
+/**
+ * Signal used to trigger transaction rollback after test completes
+ */
+class RollbackSignal extends Error {
+  constructor() {
+    super("RollbackSignal");
+    this.name = "RollbackSignal";
+  }
+}
+
+/**
+ * Runs a test function inside a transaction that auto-rollbacks.
+ *
+ * The transaction implements the same interface as Kysely<Database>,
+ * so it can be passed to context builders and used for all queries.
+ * After the test completes, the transaction is rolled back, providing
+ * instant cleanup without truncating tables.
+ *
+ * @example
+ * ```typescript
+ * test("creates user", async () => {
+ *   await withTestTransaction(getSharedDb(), async (db) => {
+ *     const user = await createTestUser(db, { email: "test@example.com" });
+ *     const ctx = createAPIContext({ db });
+ *     // ... test code
+ *   }); // Auto-rollback here
+ * });
+ * ```
+ */
+export async function withTestTransaction<T>(
+  db: Kysely<Database>,
+  testFn: (trx: Kysely<Database>) => Promise<T>,
+): Promise<T | undefined> {
+  let result: T | undefined;
+
+  try {
+    await db.transaction().execute(async (trx) => {
+      result = await testFn(trx);
+      // Force rollback by throwing after test completes successfully
+      throw new RollbackSignal();
+    });
+  } catch (e) {
+    // Swallow the rollback signal - this is expected behavior
+    if (!(e instanceof RollbackSignal)) {
+      throw e;
+    }
+  }
+
+  return result;
+}